What it is

Position sizing is the discipline of deciding how much to trade by working backwards from a risk limit and a stop distance, rather than from a gut feeling about how many shares to buy. It is the bridge that connects your risk-reward ratio and your stop-loss order to a concrete number of shares. Get it right and a losing trade costs exactly what you decided in advance; get it wrong and a single bad position can do damage that takes months to repair.

The dominant approach among professionals is fixed-fractional sizing: risk a fixed fraction of account equity on every trade - say 0.5% or 1% - regardless of how confident you feel. Because the fraction is constant, your dollar risk automatically scales up as the account grows and down as it shrinks, which is exactly the behaviour that protects you during a drawdown.

How it works

Three inputs determine your size, and they combine in one formula:

1. Account risk per trade - the fixed fraction of equity you allow yourself to lose, e.g. 1% of a 20,000 account is 200.
2. Stop distance - the gap, in price units, between your entry and your stop. This is set by the chart and the thesis, not by how much you wish to risk.
3. Position size - the number of shares, computed as dollar risk ÷ stop distance.

The critical insight is that stop distance and position size move in opposite directions for a fixed dollar risk. A wider stop forces a smaller position; a tighter stop allows a larger one. The dollar risk - the product of the two - stays pinned to your account risk cap. This is why you never first pick a share count and then place a stop: you fix the risk, read the correct stop off the chart, and let arithmetic deliver the size.

The account risk cap sits above all of this as a portfolio-level limit. Even if you have several open positions each sized to risk 1%, you may cap total simultaneous risk at, say, 3-5% of equity, so a single bad session cannot cascade. Correlated positions count together: three longs in the same sector behave like one larger position when the sector turns.

The reason the fixed fraction matters so much becomes obvious once you think about the risk of ruin - the probability of losing so much capital that you can no longer trade your edge. Risking a large fraction per trade does not just lower returns; it raises the chance that an ordinary losing streak ends the account permanently. A trader risking 10% per trade can be wiped out by a run of bad luck that a trader risking 1% would barely notice. Fixed-fractional sizing is, at its heart, a tool for surviving the inevitable bad streak so that expectancy - the average profit per trade your edge produces - has time to express itself over a large sample.

How to use it - a worked example

Suppose your account is 25,000 and your rule is 1% risk per trade, so your dollar risk is 250.

• You want to buy a stock at an entry of 50.00. Your thesis is invalidated below 47.50, so your stop distance is 2.50 per share.
• Position size = 250 ÷ 2.50 = 100 shares. Your total outlay is 5,000, but your risk is only 250.
• Now suppose the same stock is more volatile and the logical stop sits at 45.00 - a 5.00 stop distance. Size = 250 ÷ 5.00 = 50 shares. The wider stop halves the size; the risk stays 250.

Notice that in both cases your worst-case loss is identical at 250, even though the second stock is twice as volatile and you hold half as many shares. That is the whole point: the position size absorbs the difference in volatility so your risk stays constant. A common refinement is to set the stop distance from a volatility measure such as ATR rather than a fixed percentage - for example, placing the stop two ATRs below entry. A wider ATR (a more volatile symbol) then automatically produces a smaller share count, which is exactly the protective behaviour you want.

A practical checklist before every entry:

1. Confirm the fixed fraction (e.g. 1%) and compute the dollar risk from current equity.
2. Read the stop level off the chart where the thesis fails - never from a round number.
3. Divide dollar risk by stop distance to get the share count, and round down.
4. Check the position against your account risk cap and correlated-exposure rule before clicking buy.

Strengths & limits

Fixed-fractional sizing makes risk consistent across wildly different setups: a tight-stop scalp and a wide-stop swing trade can both risk exactly 1%, so your results reflect your edge rather than the accident of how big each position happened to be. It also enforces humility - volatile instruments automatically get smaller positions, which is precisely where outsized bets do the most harm.

The limits are real. Sizing math assumes your stop will actually fill near its level; a gap through the stop (overnight news, a halt) can produce a loss larger than planned, so the fixed fraction should be conservative enough to absorb the occasional overshoot. Correlation is easy to underestimate - positions that look independent can move together in a crisis, blowing through your account risk cap. And no sizing rule rescues a strategy with no edge; it only ensures that the edge, if it exists, survives long enough to compound.

Key takeaway: Fix your account risk first, read the stop off the chart, and let dollar risk ÷ stop distance set the size - wider stops mean smaller positions, never a larger loss.